Weegy: Friction can be increased by using a rougher, or coarser, material. Sand paper is one example.
Friction can be reduced by using a smoother material, this can be silk, or very very finely milled metal. [ Additionally, lubricants like oil can be used, the oil gets between where two materials make contact and effectively fills in the gaps to make rough surfaces become smoother, and allow them to slide past each other more easily. ] (More)

Weegy: a1) In a car engine, the parts rub against each other - pistons rub in the cylinders, the crankshaft rubs against its bearings, etc, etc. Sme thing in the transmission. [ The friction between those parts means that you have to apply some force to get the engine and transmission to turn. That means that some of the power of the engine is used to overcome friction instead of actually moving the car - so you use more fuel, which costs you money and releases more pollutants and global warming gases into the atmosphere. It also creates wear on the parts, which will eventually break. In the case of your pistons in the cylinders, wear will result in a bad seal, which means loss of power, and even more wasted fuel.
b1) But at the tires, friction is a good thing! The engine turns the wheels, but the thing that gets the car moving forward is the force on the ground applied by the tires. The force that can be applied is related to the friction between the tires and the ground. If you've ever stomped on the gas on a rainy day, you might have noticed that your tires started spinning without actually moving the car, because the layer of water reduces the friction between the tire and the road. The same goes for breaking - the brakes stop the wheels from spinning (as a matter of fact, they use friction to do that, too!), and friction between the tires and the road stops the car. If you try to stop on ice, there might not be enough friction, and you can lose control of your car.
a2) If you've ever hung around runners, especially long distance runners, you might have heard them talk about putting band aids over their nipples. That's because as they run, their shirts move across the skin of their chest, and there's friction between the cloth and the skin. ] (More)